1. Field of the Invention
The present invention is generally in the field of imaging devices. More specifically, the invention is in the field of imaging devices having improved dynamic range.
2. Background Art
Image sensors have broad applications in many areas. Image sensors convert a received image into representative information indicative of the received image. Examples of image sensors include solid-state image sensors, such as charge coupled devices (CCDs) and CMOS imaging devices (also known as CMOS image sensors), among others.
Image sensors are fabricated from semiconductor materials and comprise imaging arrays of light detecting, i.e., photosensitive, elements (also known as photodetectors) interconnected to generate representative information (e.g., analog signals) that corresponds to an image illuminating the device. A typical imaging array comprises a number of photodetectors arranged in a pattern, for example, a pattern comprising rows and columns. Each photodetector in the imaging array receives a portion of the light reflected from an object and received by the image sensor. Each portion is called a picture element and is typically referred to as a pixel. Each pixel provides output information representative of the luminance and/or chrominance detected by the photodetector. When considered in the context of the pattern of the photodetectors, the output information from the pixels constitutes the representative information that corresponds to the image incident upon the imaging array.
As an example, each pixel of an imaging array may provide as output information an output signal, which may, for example, be dependent upon an accumulation of charge, i.e., photo charges, according to the photoelectric effect, corresponding to the radiation intensity falling upon its detecting area defined by the physical dimensions of its corresponding photodetector. The photo charges from each pixel are converted to a charge signal, which is an electrical potential representative of the luminance and/or chrominance reflected from a respective portion of the object and received by the image sensor. The resulting charge signal or potential is read and processed by video/image processing circuitry to create a signal representation of the image.
It has been difficult to provide simultaneously wide dynamic range and very low noise in image sensor arrays. Present trends towards small pixel pitches in CMOS four-transistor (4T) technology result in loss of charge capacity as the pixel shrinks. This results in imagers with only 56 dB or less of dynamic range. This low dynamic range degrades the quality of outdoor images because there is simply not enough range in the sensor photodiode to describe both the bright and dark areas of the scene.
A conventional CMOS image sensor is typically structured as an imaging array of photodetectors, each photodetector being reset to an approximately known potential after the readout of a previous image, and in preparation for the next image. However, the performance of conventional CMOS image sensors suffers from a number of problems. For example, conventional CMOS image sensors suffer from noise associated with the process for resetting the photodiode in each pixel to a known potential after each exposure and in preparation for the next image. This noise, also referred to as reset noise or KTC noise, is often a significant source of noise in camera systems employing conventional CMOS image sensors. The reset noise is proportional to the square root of KTC, where C is the capacitance of the sense node or photodiode/source follower gate combination in a typical active pixel sensor. Reset noise is typically 30 to 40 electrons one-sigma. Reducing the capacitance of the sense node can reduce reset noise, but results in a corresponding reduction in the total charge that can be collected and therefore undesirably reduces the overall dynamic range in the camera system.
Moreover, the trend towards small pixel pitches in image sensors results in additional loss of charge capacity, further reducing dynamic range. Low dynamic range significantly degrades image quality, particularly in outdoor images, due to the limited range of the sensor's photodetector and its inability to describe both bright and dark areas of the scene in a single frame exposure.
One approach for increasing dynamic range in image sensors involves acquiring both a short exposure and a long exposure separately, each exposure stored in respective frame buffers, the short exposure being suitable for capturing bright areas of a scene, and the long exposure being suitable for capturing the dark areas of the scene. Thereafter, the image stored into the short exposure frame buffer and the image stored into the long exposure frame buffer could be integrated into a single image with improved dynamic range. However, the significantly increased costs associated with additional frame buffers renders this approach impractical for many applications.
Three-transistor (3T) pixels can be constructed with large capacities (e.g., 15,000 to 35,000 photons or more), but the higher noise floor of 25 to 35 electrons (converted photons) results in a dynamic range of 60 dB at best. However, in low light situations, the higher noise floor and higher dark current of the 3T pixel result in greatly degraded performance compared to 4T pixels.
In higher cost systems, such as those used for security applications, very high dynamic range images can be compounded by collecting multiple full-frame images and different exposures. These outputs are stored in a full-frame memory buffer and a very high dynamic image is created by software. However, the required buffer memory cost is comparable to the cost of the image sensor, and this cost is not easily borne by low-end consumer applications, such as cellular phone cameras. However, in higher end camera phones, substantial improvements in overall performance can be enabled by the use of a frame buffer in the camera.
Four-transistor (4T) pixels with pitches of about 2.25 micron to 2.8 micron struggle to have 6000 electrons of capacity while maintaining a noise floor of about 10 electrons. This lower noise floor results and the lack of significant dark signal noise results in three to four times improvement in low light performance when compared to 3T technology. However, the total dynamic range remains at 56 dB, and outdoor images are inferior to larger pixel cameras.
What is needed is a low cost solution that retains the low light performance and small form factor of the small pixel pitch but has a substantial (e.g., 2×) improvement in dynamic range for outdoor images. Accordingly, there is a strong need in the art for a cost-effective wide dynamic range image sensor.